Delay pickoff circuit

ABSTRACT

A delay pickoff circuit initiates the generation of the ramp voltage of a delayed sweep generator of an oscilloscope in response to a comparison of an adjustable voltage from a delay time multiplier in a voltage comparator with the ramp voltage of a delaying sweep generator. The circuit utilizes a delaying sweep start gate voltage, which initiates the generation of the ramp voltage of the delaying sweep generator, to cause starting of the delayed sweep generator with less time delay after the start of the delaying sweep generator than is possible in prior circuits. This delaying sweep start gate voltage is employed to supply current to a tunnel diode which causes the tunnel diode to change from its low to high voltage state to thereby cause production of a delayed sweep start gate voltage when this operation of the tunnel diodes is enabled by a signal from the comparator as a result of setting the delay time multiplier at its zero delay setting. Current derived from the delaying sweep start voltage also conditions the tunnel diode for a similar change as a result of a signal from the comparator produced by a voltage comparison when the multiplier is set for substantially greater delays. A voltage derived from the delaying sweep start gate voltage and having a lesser rise time than the delaying sweep start gate voltage is employed to supply a part of this current to the tunnel diode in order to smooth the transition between these two conditions of operation of the tunnel diode.

United States Patent De Vey et al.

[451 May 16, 1972 [54] DELAY PICKOFF CIRCUIT [72] Inventors: William J.De Vey, Beaverton; Lester L.

Larson, Portland, both of Greg.

[73] Assignee: Telttronix, lnc., Beaverton, Oreg.

[22] Filed: Mar. 17, 1971 211 Appl. No.: 125,211

Primary Examiner-Donald D. Forrer Assistant Examiner-Harold A. DixonAt10rneyBuckhorn, Blore, Klarquist & Sparkman [57] ABSTRACT A delaypickoff circuit initiates the generation of the ramp DELAYI NG SWEEPGENERATOR voltage of a delayed sweep generator of an oscilloscope inresponse to a comparison of an adjustable voltage from a delay timemultiplier in a voltage comparator with the ramp voltage of a delayingsweep generator. The circuit utilizes a delaying sweep start gatevoltage, which initiates the generation of the ramp voltage of thedelaying sweep generator, to cause starting of the delayed sweepgenerator with less time delay after the start of the delaying sweepgenerator than is possible in prior circuits. This delaying sweep startgate voltage is employed to supply current to a tunnel diode whichcauses the tunnel diode to change from its low to high voltage state tothereby cause production of a delayed sweep start gate voltage when thisoperation of the tunnel diodes is enabled by a signal from thecomparator as a result of setting the delay time multiplier at its zerodelay setting. Current derived from the delaying sweep start voltagealso conditions the tunnel diode for a similar change as a result of asignal from the comparator produced by a voltage comparison when themultiplier is set for substantially greater delays. A voltage derivedfrom the delaying sweep start gate voltage and having a lesser rise timethan the delaying sweep start gate voltage is employed to supply a partof this current to the tunnel diode in order to smooth the transitionbetween these two conditions of operation of the tunnel diode.

7 Claims, 2 Drawing Figures 64 74 DELAYED sweep 0 GENERATOR Patntecl May16, 1972 3,663,832

DELAY! NG SWEEP 74 DELAYED A SWEEP o 72 GENERATOR k/ TRIGGER I 341DELAYING swEEP j START GATE 44 M DELAYING swEEP Y| GATE /J/\ DELAY! NGswEEP I 68 T I DELAY GATE I 86 88 DELAYED SWEEP START GATE j' DELAYEDswEEP GATE )4 DELAYED SWEEP I SLOW GATE J94 FIG. 2

LLIAM J. DEVEY LESTER L... LARSON INVENTORS.

BUCKHORN, BLORE, KLAR QUIST & SPARKMAN ATTORNEYS DELAY PICKOFF CIRCUITBACKGROUND OF THE INVENTION Precision Oscilloscopes usually have twoseparate horizontal sweep generator circuits each capable of generatinga sweep voltage ramp. One of these, called the delaying sweep generator,has is operation initiated by a trigger pulse from a trigger generatorand is employed, in conjunction with a voltage comparator and a timedelay multiplier providing an adjustable voltage, to delay the start ofgeneration of a ramp voltage by the other sweep generator, called thedelayed sweep generator. The adjusted voltage from the sweep timemultiplier is compared in the voltage comparator with the ramp voltagegenerated by the delaying sweep generator to produce an output from thecomparator which initiates the operation of the delayed sweep generatorwhen the ramp voltage of the delaying sweep generator equals theadjusted voltage. One important use of the delayed sweep is to enable asmall portion of a waveform, which may be spaced any desired distancealong the waveform from the portion of such waveform corresponding intime to the trigger pulse, to be selected and magnified horizontally soas to extend across a major portion of the screen of the oscilloscope.

Prior circuits have depended solely upon the time delay afforded by thecomparison of the adjustable voltage from the time delay multiplier withthe voltage of the ramp generated by the delaying sweep generator todelay the start of the delayed sweep generator. There is always asubstantial time interval between the production of a sweep start gatevoltage as a result of the delivery of a trigger pulse to a sweepgenerator circuit and the actual start of the ramp voltage of such sweepgenerator. Since the operation of the delayed sweep generator was notinitiated in the prior circuits until the voltage ramp of the delayingsweep generator was actually being generated, it was impossible to causethe delayed sweep generator to start until after termination of suchtime interval even though the delay time multiplier was set for zerodelay. A portion of the waveform immediately following the portion ofthe waveform corresponding to the trigger pulse could therefor not bedisplayed when employing the delayed sweep. The portion of the waveformwhich could not be displayed during the delayed sweep in prior circuitscan become a material portion of the total waveform being displayed by ahigh speed oscilloscope capable of displaying waveforms of high rise andfall times or of high frequency.

SUMMARY OF THE INVENTION In accordance with the present invention a fastrise time sweep start gate voltage for the delaying sweep generatorproduced in response to a trigger pulse, with very slight delay aftersuch trigger pulse, is employed to produce a similar sweep start gatevoltage for the delayed sweep generator, also with very slight delay,when the delay time multiplier is set for zero time delay. At thissetting of the delay time multiplier, the voltage comparator associatedwith the delay time multiplier furnishes an enabling signal enabling thesweep start gate voltage for the delaying sweep generator to cause suchproduction of the sweep start gate voltage for the delayed sweepgenerator. Since the time interval between the production of a sweepstart gate voltage of a sweep generator and the start of the voltageramp is essentially the same for both the delaying sweep and the delayedsweep, the delay of the start of the delayed sweep after the start ofthe delaying sweep, when the delay time multiplier is in the zero delayposition, can be made as short as 3 to 5 nanoseconds as compared toapproximately 50 to several hundred nanoseconds in prior circuits.

When the delay time multiplier is moved a small increment from its zerodelay time position, the enabling signal from the voltage comparatordecreases so as to no longer enable the delaying sweep start voltage toproduce a delayed sweep start gate voltage. The delaying sweep startgate voltage is however sent through an amplifier to produce a slowerrise time voltage and a relatively slow rise time signal derived fromthe output of the amplifier is added to the decreased enabling signalvoltage from the comparator. For small displacements of the delay timemultiplier from its zero delay position, the efiect is the same as ifthe slow rise time voltage derived from the delaying sweep start gatevoltage in the amplifier were added to the delaying sweep start gatevoltage and the resulting voltage were still enabled by the decreasedsignal from the comparator to produce the delayed sweep start gatevoltage. The result is that the production of the delaying sweep startgate voltage is delayed by the slow rise time signal.

As the delay time multiplier is moved further away from its zero delayposition, the enabling signal from the comparator prior to the start ofthe delaying sweep become too small to enable the slow rise time signalto cause production of a delayed sweep start gate voltage in the absenceof a comparison signal from the comparator, produced by comparing thevoltage from the delay sweep multiplier and the voltage from thedelaying sweep voltage ramp. The delaying sweep start gate voltage inconjunction with the slow rise time voltage becomes an enabling voltagefor such comparison signal. It is thus possible to produce a smoothtransition between the condition at zero setting of the delay timemultiplier when the signal from the comparator acts as an enablingsignal for producing a delayed sweep start gate voltage responsive tothe delaying sweep start gate voltage and the condition when thedelaying sweep start gate voltage becomes the primary enabling signalfor producing a delayed sweep start gate voltage responsive to acomparison signal from the comparator. In operation of the circuit justdescribed, the delay of the start of the delayed sweep relative to thestart of the delaying sweep can be made substantially a linear functionof the angular setting of the dial of the delay time multiplier.

It is therefor an object of the invention to provide a delay pickofi'circuit for an oscilloscope in which the minimum delay between the startof a delaying sweep voltage ramp and the start of a delayed sweepvoltage ramp is reduced to a small fraction of that of prior circuitswhile at the same time providing a delay of the start of the delayedsweep voltage ramp which is a substantially linear function of theangular position of a delay time multiplier dial.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partly schematic diagramand partly a block diagram illustrating a delay pickoff circuit inaccordance with the present invention; and

FIG. 2 is a view showing waveforms useful in explaining the operation ofthe circuit of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT A delaying sweep generator 10for producing a delaying sweep voltage ramp 12, in conjunction with avoltage comparator l4 and a delay time multiplier 16 provides a signalused to start a delayed sweep generator 18, to generate a delayed sweepvoltage ramp 20. The generation of the delayed sweep voltage ramp 20 isstarted at the expiration of a delay time determined by a comparison inthe voltage comparator 14 of the delaying voltage ramp 12 with anadjustable voltage from the delay time multiplier 16, for major delaytimes.

The generation of the ramp 12 by the delaying sweep generator isinitiated by the application of a positive trigger pulse 22 from aninput terminal 24 through a junction 26 to the anode of a delaying sweepstart tunnel diode 28 connected between the junction 26 and ground orthe other common connection. The tunnel diode 28 is normally in its lowvoltage state but has its anode connected through a diode 30 and anadjustable bias resistor 32 to a source of positive potential to receivesufficient bias current to maintain the tunnel diode 28 in its highvoltage state whenever it is changed to such high voltage state. Thepositive trigger pulse 22 causes the tunnel diode 28 to change to itshigher voltage state to produce a delaying sweep start gate voltage 34,shown in FIG. 2. The tunnel diode can be reset to its low voltage stateat any time by supplying a positive voltage through a terminal 36 to thegate of an NPN transistor 38 having its emitter connected to ground andits collector connected to a junction 40 between the resistor 32 and thediode 30. The transistor 38 is normally turned off but the applicationof a positive voltage to the terminal 36 will cause the transistor 38 toturn on and short circuit the tunnel diode 28 to cause it to return toits low voltage state.

The delaying sweep start gate voltage produced by the tunnel diode 28has a fast rise time leading edge 42 and is delivered to the delayingsweep generator to initiate the operation of this sweep generator. Thedelaying sweep start gate 34 is employed in the delaying sweep generator10 to produce a delaying sweep gate voltage 44 which also has a fastrise time leading edge 46 which actually starts the production of theramp voltage 12. There is a substantial delay between the leading edge42 of the delaying sweep start gate voltage 34 and the leading edge 46of the delaying sweep gate voltage 44. The reason for this delay is thatthe delaying sweep start gate voltage 34 is first employed to actuate adelaying sweep multivibrator, the output of which is then amplified toproduce the delaying sweep gate voltage 44 which actually starts thedelaying sweep generator. This delay may be a substantial portion of thelength of the delaying sweep voltage ramp 12. The vertical input signalto the oscilloscope providing the waveform to be displayed can, however,be sent through a delay line so that it is delayed for the same timeinterval as the delayed sweep gate voltage 44 delayed with respect tothe delaying sweep start gate voltage 34 so that the trace of thewaveform to be displayed by the oscilloscope starts at the same time asthe delaying sweep ramp voltage 12.

Prior delaying pickoff circuits have exclusively employed a rise involtage of the delaying sweep ramp voltage 12 to produce a comparisonsignal in a comparator and then have employed this comparison signal toproduce a delayed sweep start gate voltage. Since the circuits of adelayed sweep generator are substantially the same as those of adelaying sweep generator, there is a delay between the leading edge ofthis delayed sweep start gate voltage of the delayed sweep generator andthe start of the delayed sweep voltage ramp of the delayed sweepgenerator similar to the delay between the leading edge 42 of thedelaying sweep start gate voltage 34 and the start of the delaying sweepramp voltage 12 of the delaying sweep generator 10 discussed above. Thismeans that the portion of the input waveform immediately following thebeginning of the delaying sweep ramp voltage cannot be displayed at thehorizontal magnification afforded by the delayed sweep voltage ramp.

The comparator 14 of the present circuit has a movable contact 48 and itwill be understood that this contact is moved by a manual controlelement providing an accurately calibrated indication of the position ofthe contact. The comparator produces a small positive enabling voltageas an output when such control element and the movable contact 48 ofsuch multiplier are in a zero time delay position. This can beaccomplished, for example, by offsetting the quiescent voltage of thedelaying sweep generator 10 from ground potential in a positivedirection to produce a constant positive output voltage from thecomparator 14 when the delaying sweep voltage ramp 12 is not beinggenerated by the sweep generator 10. This output voltage causes currentflow to ground through a resistor 50 to a junction 51 and then through adiode 52 and a delay gate tunnel diode 54 having its anode connected toa junction 56 between the diode 52 and the tunnel diode 54. This currentplus an additional current through an adjustable bias resistor 58 from asource of positive potential is not sufficient to change the tunneldiode 54 from a low voltage state to a high voltage state but issufficient to enable the delaying sweep start gate voltage produced atthe junction 26, when the tunnel diode 28 is changed from its lowvoltage state to its high voltage state, to cause the tunnel diode 54 tochange to its high voltage state as a result of current flow through aresistor 60 connected between the junctions 26 and $6. The currentthrough resistors 50 and 58 is, however, sufiicient to hold the tunneldiode 54 in its high voltage state. The tunnel diode 54 can be reset toits low voltage state at any time by applying a positive voltage to aterminal 62 connected to the base of a normally turned off NPNtransistor 64 having its emitter connected to ground and its collectorconnected to the junction 51. This turns on the transistor 64 to shortcircuit the tunnel diode 54.

The change of a tunnel diode 54 to its high voltage state produces afast rise time delay gate voltage 68 shown in FIG. 2. Leading edge 70 ofthis delay gate voltage is produced with very little time delay afterthe leading edge 42 of the delaying sweep start gate voltage 34, thisslight delay being primarily caused by the charging of the smalldistributed capacitances of the elements connected to the junction 56through a resistor 60. The delay gate voltage 68 causes current to flowthrough a resistor 72 through a junction 74 and a delayed sweep gatetunnel diode 76 connected between the junction 74 and ground to causethis tunnel diode to change from a low voltage state to a high voltagestate. The tunnel diode 76 is also supplied with the bias current from apositive source of potential through an adjustable bias resistor 77 anda diode 78 which is not sufficient to cause the tunnel diode 76 tochange from a low voltage state to a high voltage state but issufficient to hold such tunnel diode in its high voltage state. An NPNtransistor 80 has its emitter connected to ground and its collectorconnected to a junction 82 between the resistor 76 and diode 78. Thetransistor 80 is normally turned off but can be turned on by a positivevoltage supplied to the terminal 84 to short circuit the tunnel diode 76and return it to its low voltage state.

The change of the tunnel diode 76 from its low voltage state to a highvoltage state due to current through the resistor 72 caused by thechange of the tunnel diode 54 to its high voltage state, produces adelayed sweep start gate voltage 86, shown in FIG. 2, which also has afast rise time leading edge 88. The fast rise time leading edge 88 ofthe delayed sweep start gate voltage 86 occurs after a very slight delayfrom the production of the leading edge 70 of the delay gate 68.

The delayed sweep gate voltage 86 is delivered to the delayed sweepgenerator 18 and produces a delayed sweep gate voltage 90. This delayedsweep gate voltage also has a fast rise time leading edge 91 which isdelayed with respect to the leading edge 88 of the delayed sweep startgate voltage 88 by a time interval which is substantially the same asthe time interval between the leading edge 42 of the delaying sweepstart gate voltage 34 and the leading edge of the delaying sweep gatevoltage 44. The delayed sweep voltage ramp 20 starts at the same time asthe leading edge 91 of the delayed sweep gate voltage 90 and thereforthis delayed sweep voltage ramp starts with very slight delay after thestart of the delaying sweep ramp voltage 12. The delay between the startof the delaying sweep ramp voltage 12 and the start of the delayed sweepramp voltage 20 is substantially the same as the delay between leadingedge of the trigger pulse 22 and leading edge 88 of the delayed sweepstart gate voltage 86. This delay may be made as low as 3 to 5nanoseconds. The enabling voltage from the comparator 14 when thecontact 48 of the delay time multiplier is in its zero delay position,thus enables a delayed sweep ramp voltage 20 to be started almostimmediately after the start of a delaying sweep ramp voltage 12 so thatthe portion of the waveform being displayed immediately after the startof the delaying sweep ramp voltage 12 can thus be displayed during thedelayed sweep ramp voltage 20 generated by the delayed sweep generator.

If the contact 48 of the delay time multiplier is moved a smallincrement away from its zero delay time position, the enabling voltagefrom the comparator 14 prior to the start of the delaying sweep rampvoltage 12 is decreased and insufficient current is delivered to thetunnel diode 54 to enable this tunnel diode to be changed from a lowvoltage state to a high voltage state by the delaying sweep start gatevoltage 34 from the tunnel diode 28. This condition exists until thevoltage of the delaying sweep ramp voltage 12 has increased sufficientlyto produce a positive comparison voltage output from the comparator.With the circuit thus far described, the result would be that the tunneldiode 54 would not change to its high voltage condition until after thedelaying sweep ramp voltage 12 has started and reached a voltagesufiicient to again increase the output voltage from the comparator 14.

To correct this condition just described, an amplifier 92 is connectedto receive the delaying sweep start gate voltage 34 and produces a slowgate voltage 94 having a low rise time as compared to the delaying sweepstart gate voltage as an output. This output is connected through aresistor 96 to the junction 40 so that a resulting current havingsubstantially the fonn of the slow gate voltage 94 is also delivered tothe tunnel diode 54. This additional current causes tunnel diode 54 tochange from its low voltage state to its high voltage state atprogressively later time as the movable contact 48 of the delay timemultiplier is moved in small increnemts away from its zero delay timeposition. For such small increments the slope of the slow gate voltage94 is sufficient to produce an increasing current through the resistor96 which more than compensates for the decrease in current through theresistor 50 resulting from such movement of the movable contact 48.

A conventional positive holdofi voltage from the delaying sweepgenerator is applied to the terminals 36 and 62 at the end of thedelaying sweep gate 44 to reset the tunnel diodes 28 and 54 and thisvoltage is maintained for a hold off period of sufficient time to enableall of the circuits of the delaying sweep generator including the usualintegrating circuit of such generators to reach a quiescent state. Asimilar hold ofi' voltage from the delayed sweep generator is applied tothe terminal 84 connected to the gate of the transistor 80 to reset andhold off the tunnel diode 76. This tunnel diode is employed to provideanother delay mode of operation in which the delayed sweep generator isinitiated by another trigger pulse after the delay by circuits which arenot shown, but is included in the disclosure in order to show the reasonfor the minimum time delay realizable in a practical oscilloscopebetween the start of the delaying sweep ramp voltage and the delayedsweep ramp voltage.

From the above description of the preferred embodiment, it is apparentthat for settings of the delay time multiplier 16 at or near zero timedelay, a signal from the comparator 14 which decreases with increase ofthe delay time setting, enables a signal including the delaying sweepstart gate voltage 42 plus a lower rise time voltage 96 derived fromsuch delaying sweep start gate voltage 42 to produce a delayed sweepstart gate voltage 91. As the time delay setting of the comparator isfurther increased a comparison voltage again increases the signal fromthe comparator. There is a crossover point at which the delaying sweepstart gate voltage 44 plus the slow rise time voltage 94 becomes theenabling signal for the com parison signal from the comparator. The slowgate voltage 94 smoothes the transition between the operation of thecircuit at the zero delay time setting of the delay time multiplier andsettings of this delay time multiplier remote from the zero delay timesetting to provide a delay time which is a substan tially linearfunction of the delay time setting of the delay time multiplier.

We claim:

1. A delay pickofi circuit for an oscilloscope having a delaying sweepgenerator, a delayed sweep generator, a delaying sweep starting meansresponsive to a trigger pulse for initiating the operation of saiddelaying sweep generator to generate a delaying sweep ramp voltage, anda delay time multiplier provided with a control element movable from azero delay position through a range of delay positions for supplying anadjustable voltage, said delay pickoff circuit comprising:

a voltage comparator for comparing the voltage of said ramp with saidadjustable voltage and for providing an enabling signal when saidcontrol element is in said zero delay position and an actuating signalwhen said control element is in a delaying position remote from saidzero delay position;

delayed sweep starting means enabled by said enabling signal foractuation by said delaying sweep starting means to initiate theoperation of said delayed sweep generator to generate a delayed sweepramp voltage when said control element is in said zero delay position;

and enabling means for causing said delayed sweep starting means to beenabled by a signal from said delaying sweep generator starting means soas to be actuated by said actuating signal for initiating the operationof said delayed sweep generator to generate a delayed sweep ramp voltagewhen said control element is in said position remote from said zerodelay position.

2. The delay pickoff circuit of claim 1 in which said enabling meansincludes:

transition means responsive to a signal from said delaying sweepstarting means for producing and transmitting a signal to said delayedsweep starting means for smoothing the transition between the enablingof said delayed sweep starting means by said signal from said comparatorwhen said control element is in said zero delay position and theenabling of said delayed sweep starting means by said signal from saiddelaying sweep starting means when said control means is in saidposition remote from said zero delay position.

3. The delay pickoff circuit of claim 1 in which:

said voltage comparator decreases said enabling signal as said controlelement is progressively moved through small increments from said zerodelay position;

said delaying sweep starting means includes means for producing a fastrise time delaying sweep starting gate voltage in response to saidtrigger pulse;

and said transition means includes means for producing a lower rise timesignal in response to said delaying sweep starting gate voltage foractuating said delayed sweep starting means with progressively largerdelays corresponding to said increments.

4. The delay pickotf circuit of claim 1 in which:

said delayed sweep starting means includes a tunnel diode for producinga fast rise time signal for initiating said operation of said delayedsweep generator;

and said delaying sweep starting means produces a fast rise time signalfor initiating said operation of said delaying sweep generator and fortransmission to said tunnel diode to cause said tunnel diode to changefrom a low voltage state to a high voltage state.

5. The delay pickoff circuit of claim 4 which also includes:

transition means for producing a slower rise time signal from said fastrise time signal produced by said delaying sweep starting means andtransmitting said slower rise time signal to said tunnel diode forsmoothing the transition between the enabling of said delayed sweepstarting means by said signal from said comparator when said controlelement is in said zero delay position and the enabling of said delayedsweep starting means by said delaying sweep starting means when saidcontrol means is in said position remote from said zero delay position.

6. The delay pickolf circuit of claim 4 in which:

said delaying sweep starting means includes a tunnel diode for producingsaid fast rise time signal for transmission to the first mentionedtunnel diode.

7. The delay pickofi' circuit of claim 4 which also includes:

a second tunnel diode receiving a fast rise time signal from the firstmentioned tunnel diode and producing a fast rise time delayed sweepstart gate signal for initiating said operation of said delayed sweepgenerator.

i t il i I!

1. A delay pickoff circuit for an oscilloscope having a delaying sweepgenerator, a delayed sweep generator, a delaying sweep starting meansresponsive to a trigger pulse for initiating the operation of saiddelaying sweep generator to generate a delaying sweep ramp voltage, anda delay time multiplier provided with a control element movable from azero delay position through a range of delay positions for supplying anadjustable voltage, said delay pickoff circuit comprising: a voltagecomparator for comparing the voltage of said ramp with said adjustablevoltage and for providing an enabling signal when said control elementis in said zero delay position and an actuating signal when said controlelement is in a delaying position remote from said zero delay position;delayed sweep starting means enabled by said enabling signal foractuation by said delaying sweep starting means to initiate theoperation of said delayed sweep generator to generate a delayed sweepramp voltage when said control element is in said zero delay position;and enabling means for causing said delayed sweep starting means to beenabled by a signal from said delaying sweep generator starting means soas to be actuated by said actuating signal for initiating the operationof said delayed sweep generator to generate a delayed sweep ramp voltagewhen said control element is in said position remote from said zerodelay position.
 2. The delay pickoff circuit of claim 1 in which saidenabling means includes: transition means responsive to a signal fromsaid delaying sweep starTing means for producing and transmitting asignal to said delayed sweep starting means for smoothing the transitionbetween the enabling of said delayed sweep starting means by said signalfrom said comparator when said control element is in said zero delayposition and the enabling of said delayed sweep starting means by saidsignal from said delaying sweep starting means when said control meansis in said position remote from said zero delay position.
 3. The delaypickoff circuit of claim 1 in which: said voltage comparator decreasessaid enabling signal as said control element is progressively movedthrough small increments from said zero delay position; said delayingsweep starting means includes means for producing a fast rise timedelaying sweep starting gate voltage in response to said trigger pulse;and said transition means includes means for producing a lower rise timesignal in response to said delaying sweep starting gate voltage foractuating said delayed sweep starting means with progressively largerdelays corresponding to said increments.
 4. The delay pickoff circuit ofclaim 1 in which: said delayed sweep starting means includes a tunneldiode for producing a fast rise time signal for initiating saidoperation of said delayed sweep generator; and said delaying sweepstarting means produces a fast rise time signal for initiating saidoperation of said delaying sweep generator and for transmission to saidtunnel diode to cause said tunnel diode to change from a low voltagestate to a high voltage state.
 5. The delay pickoff circuit of claim 4which also includes: transition means for producing a slower rise timesignal from said fast rise time signal produced by said delaying sweepstarting means and transmitting said slower rise time signal to saidtunnel diode for smoothing the transition between the enabling of saiddelayed sweep starting means by said signal from said comparator whensaid control element is in said zero delay position and the enabling ofsaid delayed sweep starting means by said delaying sweep starting meanswhen said control means is in said position remote from said zero delayposition.
 6. The delay pickoff circuit of claim 4 in which: saiddelaying sweep starting means includes a tunnel diode for producing saidfast rise time signal for transmission to the first mentioned tunneldiode.
 7. The delay pickoff circuit of claim 4 which also includes: asecond tunnel diode receiving a fast rise time signal from the firstmentioned tunnel diode and producing a fast rise time delayed sweepstart gate signal for initiating said operation of said delayed sweepgenerator.